See U.S. Department of Transportation, Bureau of Transportation Statistics,
pp. 273-280, National Transportation Statistics 1999 for detailed discussion of modal injury data.

SOURCE: Data compiled from various government agencies, as
cited in U.S. Department of Transportation, Bureau of Transportation Statistics,
National Transportation Statistics 1999, table 3-2, available
at: http://www.bts.gov/ntda/nts/nts.html.